In a generic method of this type, also more narrowly designated as “calculated differential thermal analysis” method or c-DTA method, a sample disposed in a temperable test space is tempered according to an essentially linear temperature program, extending from a start temperature to an end temperature. From the result of measuring the trial temperature at a number of measurement time points during tempering, a so-called c-DTA signal is calculated as the difference between measured trial temperature and a reference temperature calculated according to a temperature curve model.
A c-DTA method of this type is known from DE 199 34 448 A1. In this known method, a sample is heated or cooled according to a linear temperature program (that is, with an essentially stable temperature modification rate), extending from a start temperature TS to a final temperature TF. During this tempering, the sample temperature is measured and stored by means of a sample thermal element.
From the start temperature TS, the final temperature TF and the relevant time points, that is a start time tS and end time tF, a median tempering rate (heating rate or cooling rate) β can be calculated as follows:β=(TF−TS)/(tF−tS)  (equation 1)
According to the linear temperature program, a temperature curve model and/or a “calculated sample temperature” TC can be indicated in simple manner for each time point t as follows:TC(t)=TS+β×(t−tS)  (equation 2)
A DTA signal c-DTA(t) required as the result of the differential thermal analysis is then calculated as the difference between the sample temperature T(t) measured by means of the sample thermal element and the reference temperature TC(t) calculated according to equation 2:c-DTA(t)=TC(t)−T(t)
A c-DTA signal not equal to zero indicates thermal effects in the sample, for example heat tones (changes in enthalpy) on account of phase transitions or the like. It is effects like these that are primarily of particular interest in the context of a differential thermal analysis and are recognizable in the course of the c-DTA signal.
It should be stated at this point that in differential thermal analyses of the type that is of interest here, other parameters or sample features (e.g., mass modifications of the sample, etc.) besides the sample temperature can also be measured in time-dissolved manner (and stored) during tempering, approximately at the same measuring time points as in the sample temperature measurements.
Although such additional measurements, which can be selected or concretely configured by referring to the general state of the art concerning differential thermal analyses, are also preferred in the context of the inventive method, this is still of rather secondary importance for the invention. The crux of the invention is the manner in which the c-DTA signal is ascertained on the basis of the time-dissolved sample temperature measured during tempering.
A disadvantage in the method known from DE 199 34 448 A1 is that a c-DTA signal not equal to zero can also result from a non-linearity of the temperature program, that is, a temperature that changes non-linearly over time. Thus, in the known method, a non-linear temperature program unavoidable in practice leads to a corresponding distortion of the c-DTA signal, which ideally is intended only to represent or reveal the thermal effects in the sample.
In order primarily to depict the thermal effects with the c-DTA signal, the main consideration, as likewise disclosed in DE 199 34 448 A1, is to assess the measured sample temperature or to calculate the c-DTA signal only for a (relatively small) segment of the entire duration of the temperature program. In this case, the influence of non-linearity of the (entire) temperature curve is rather limited. Disadvantageously then, however, the c-DTA signal is obtained only for this relatively small partial area.